Secondary battery

ABSTRACT

A secondary battery is comprising an organic electrolytic solution contained in a casing and, disposed therein, a positive electrode comprised of a lithium-containing composite metal oxide as a cathode active material and a negative electrode comprised of a carbonaceous material as an anode active material, wherein the positive and negative electrodes are separated through a separator disposed therebetween and wherein the organic electrolytic solution has a water content of from 5 ppm to 450 ppm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a secondary battery. More particularly,the present invention is concerned with a secondary battery comprisingan organic electrolytic solution having disposed therein a positiveelectrode comprised of a lithium-containing composite metal oxide as acathode active material and a negative electrode comprised of acarbonaceous material as an anode active material, wherein the organicelectrolytic solution has a water content of from 5 ppm to 450 ppm, sothat the secondary battery has excellent current efficiency, cyclecharacteristics, storage characteristics and safety.

2. Discussion of Related Art

In recent years, various non-aqueous type secondary batteries have beenproposed as compact, light weight batteries to be advantageouslysubstituted for conventional acid-lead batteries and nickel-cadmiumbatteries. Among these proposed secondary batteries, new type secondarybatteries using a composite metal oxide comprised mainly of Li and Co asa cathode active material and using a carbonaceous material as an anodeactive material, have been attracting attention in the art. Such newtype secondary batteries are disclosed in, for example, Japanese PatentApplication Laid-Open Specification Nos. 62-90,863 (corresponding toU.S. Pat. No. 4,668,595), 63-121,260, and 3-49,155 (corresponding toU.S. Pat. No. 4,943,497).

In conventional non-aqueous type secondary batteries, metallic lithiumor a lithium alloy has been used as an anode active material. Suchconventional secondary batteries using metallic lithium or the like asan anode active material, are satisfactory with respect to compactnessin size and lightness in weight, but have various problems in practicaluse thereof, such as a lowering of cycle characteristics and a loweringof storage characteristics due to the deposition of dendrites, anoccurrence of internal short-circuiting due to the breakage of aseparator by deposited dendrites, and a safety problem ascribed to thehigh reactivity of metallic lithium.

In contrast, with respect to the above-mentioned new type secondarybatteries using a carbonaceous material as an anode active material, nodeposition of dendrites occurs, so that excellent cycle characteristicsand storage characteristics can be enjoyed, and, in addition, thecarbonaceous material does not have a high reactivity, unlike metalliclithium, so that extremely high safety can be achieved. Especially, ithas been expected that the combined use of a carbonaceous material anodeand a lithium-containing composite metal oxide cathode would provide asecondary battery exhibiting high voltage and high capacity.

However, actually, such non-aqueous type secondary batteries using alithium-containing composite metal oxide as a cathode active material incombination with a carbonaceous material as an anode active material,frequently suffer not only from various problems in performance, such asa lowering of current efficiency and a lowering of cyclecharacteristics, but safety problems also develop due to the occurrenceof a rise in internal pressure.

SUMMARY OF THE INVENTION

In these situations, the present inventors have made extensive andintensive studies with a view toward solving various problemsaccompanying the above-mentioned non-aqueous type secondary battery,namely, the problems in performance, such as the lowering of currentefficiency and lowering of cycle characteristics, as well as the safetyproblems caused by the occurrence of a rise in internal pressure. As aresult, it has unexpectedly been found that when the water content of anorganic electrolytic solution is suppressed to 450 ppm or less, theabove-mentioned non-aqueous type secondary battery can exhibit not onlyexcellent performance characteristics but also high safety. The presentinvention has been completed, based on this novel finding.

Accordingly, it is an object of the present invention to provide asecondary battery using a lithium-containing composite metal oxide as acathode active material and a carbonaceous material as an anode activematerial, which is excellent in not only current efficiency and cyclecharacteristics, but also storage characteristics and safety.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic vertical cross-sectional view of the secondarybattery of the present invention which was prepared in Example 1; and

FIG. 2 is a diagrammatic cross-sectional view, taken along line II--IIof FIG. 1, showing the spirally wound states of a positive electrode, aseparator and a negative electrode.

DETAILED DISCUSSION

The foregoing and other objects, features and advantages of the presentinvention will be apparent from the following detailed description takenin connection with the accompanying drawing and the appended claims.

According to the present invention, there is provided a secondarybattery comprising:

a casing,

an organic electrolytic solution contained in the casing, the solutionhaving a water content of from 5 to 450 ppm,

a positive electrode comprising a lithium-containing composite metaloxide as a cathode active material,

a negative electrode comprising a carbonaceous material as an anodeactive material, and

a separator disposed between the positive and negative electrodes,

the positive and negative electrodes and separator being disposed in theorganic electrolytic solution.

In the secondary battery of the present invention, the water content ofthe organic electrolytic solution contained in the battery isparticularly important and must be in the range of from 5 ppm to 450ppm, preferably from 15 ppm to 300 ppm.

When the water content exceeds 450 ppm, gases including hydrogen gas arelikely to be produced within the battery to thereby disadvantageouslycause a rise in internal pressure, leading to an occurrence of expansionof the battery. On the other hand, for controlling the water content ofthe organic electrolytic solution contained in the battery to less than5 ppm, not only is it necessary to conduct dehydrating treatment for anextremely long period of time but also an extremely strict control ofmoisture in assembling a battery is needed, which disadvantageouslyimposes various difficult problems.

The water content in the range specified in the present invention can becontrolled by treating the organic electrolytic solution with adehydrating agent, such as a molecular sieve or the like. With respectto various parts, such as, the positive and negative electrodes, theseparator, etc., the water content thereof can be controlled, forexample, by using the parts which have preliminarily been subjected to adrying treatment, or by inserting a parts-drying step in the batteryassembling procedure prior to the step for impregnating the parts withthe organic electrolytic solution.

It is noted that for achieving the water content ranging from 5 ppm to450 ppm with respect to the organic electrolytic solution contained inthe secondary battery of the present invention, extremely strict dryingconditions need not be used in the battery assembling procedure.Especially when the above-mentioned parts-drying step is inserted priorto the step for impregnating the parts with the organic electrolyticsolution, control of the water content of the organic electrolyticsolution within the range specified in the present invention can beachieved even by handling the parts under ordinary ambient conditionsprior to the parts-drying step. This has a great advantage from apractical point of view.

The lithium-containing composite metal oxide used in the secondarybattery of the present invention is a compound having a lamellarstructure and having the capability to electrochemically intercalate anddeintercalate Li ions. Although such composite metal oxides are notspecifically limited, the following compounds can be mentioned aspreferred examples of composite metal oxides: LiCoO₂ disclosed inJapanese Patent Application laid-Open Specification No. 55-136,131(corresponding to U.S. Pat. No. 4,357,215); Li_(x) Co_(y) N_(z) O₂wherein N is at least one member selected from the group consisting ofAl, In and Sn, and x, y and z are, respectively, defined by 0.05≦x≦1.10,0.85≦y≦1.00, and 0.001≦z≦0.10, as disclosed in Japanese PatentApplication Laid-Open Specification No. 62-90,863 (corresponding to U.S.Pat. No. 4,668,595); Li_(x) Ni_(y) Co.sub.(1-y) O₂ wherein 0<x≦1, and0≦y<0.50, as disclosed in Japanese Patent Application Laid-OpenSpecification No. 3-49,155; and Li_(x) MnO₂.

These compounds can be easily obtained by the calcination reaction of alithium compound, such as lithium hydroxide, lithium oxide, lithiumcarbonate, lithium nitrate or the like, with an oxide, hydroxide,carbonate, nitrate or the like of a predetermined metal and, if desired,with other metal compounds.

These composite metal oxides can, as anode active materials, exhibitexcellent properties, such as high voltage and high capacity, ascompared to those of conventional active materials. Especially, acompound represented by Li_(x) Co_(y) N_(z) O₂ wherein N is at least onemember selected from the group consisting of Al, In and Sn, and x, y andz are, respectively, defined by 0.05≦x≦1.10, 0.85≦y≦1.00, and0.001≦z≦0.10, has excellent cycle characteristics etc., so that thiscompound can be advantageously used as a cathode active material.

The carbonaceous material used in the secondary battery of the presentinvention is not particularly limited, and various types of carbonaceousmaterials can be used, for example, a carbon or graphite material havinga large surface area which is disclosed in Japanese Patent ApplicationLaid-Open Specification No. 58-35,881 (corresponding to U.S. Pat. No.4,617,243), a calcination carbonized product of a phenolic resin whichis disclosed in Japanese Patent Application Laid-Open Specification No.58-209,864, a calcination carbonized product of a condensed polycyclichydrocarbon compound which is disclosed in Japanese Patent ApplicationLaid-Open Specification No. 61-111,907 (corresponding to U.S. Pat. No.4,725,422), and a carbonaceous material disclosed in Japanese PatentApplication Laid-Open Specification No. 62-90,863, which has a BETspecific surface area A (m² /g) in the range of 0.1<A<100, and has acrystal thickness Lc (Å) in the X-ray diffraction and a true densityρ(g/cm³), respectively satisfying 10<Lc<120ρ189 and 1.70<ρ<2.18. Amongthese, the last-mentioned carbonaceous material has high capacity andexcellent cycle characteristics and can be advantageously used in thepresent invention.

A method for constructing positive and negative electrodes respectivelyfrom the above-mentioned cathode and anode active materials is notspecifically limited. However, it is preferred that a respective activematerial be dispersed in a solution of an organic polymer as a binderdissolved in a solvent therefor and the resultant dispersion be appliedto a substrate by coating, because fabrication of the active materialinto a thin film having a large area can be achieved. In the presentinvention, it is further preferred that at least one of the positive andnegative electrodes be in the form of a coating composition formed on ametallic current collector, in which the coating composition comprisesan active material corresponding to the respective electrode and abinder, wherein the binder is distributed in the coating at a binderdistribution coefficient of from 0.5 to 5.0, preferably from 0.75 to2.5, more preferably from 0.75 to 2.0. The use of a binder distributioncoefficient in the above range is advantageously effective forpreventing a lowering of the strength of the coating composition andpreventing a failure in contact between the active material particles,thereby improving high temperature characteristics of the secondarybattery of the present invention.

The term "binder distribution coefficient" used in the present inventionis a coefficient obtained by the measuring method described below. Thiscoefficient is defined as a ratio of the amount of a binder present inthe coating composition layer from its surface to the depth of 10 μm tothe amount of a binder present in the coating composition layer from theinterface between the metallic current collector and the coatingcomposition layer to the level of 10 μm above the collector.

Measurement For Determining The Binder Distribution Coefficient:

Sample:

An electrode to be measured is embedded in an epoxy resin, followed bysolidification of the resin. Cutting is made to expose a cross sectionof the electrode and the cross-section is polished before it is used asa sample for measurement. Pretreatment of the sample with, for example,osmic acid is effective for making the measurement more precise. Amethod of pretreatment may be selected arbitrarily, depending on thetype of the binder.

Measurement:

The amounts of binder respectively in the predetermined layers ofcoating composition of the cross section of the electrode are measuredby electron probe micro analysis (EPMA).

As measuring apparatus, Hitachi X-650 (manufactured by Hitachi, Ltd.,Japan) and Horiba EMAX-2200 (manufactured by Horiba Seisakusho Co.,Ltd., Japan), which are wavelength dispersion type electron probemicroanalyzers, are employed.

Calculation of The Binder Distribution Coefficient:

The binder distribution coefficient is calculated by the followingformula: ##EQU1##

When the binder distribution coefficient is less than 0.5, the surfacestrength of the coating composition disadvantageously becomes low, sothat the active material is likely to fall off from the currentcollector. On the other hand, when the binder distribution coefficientexceeds 5.0, the characteristics of the battery, in particular cyclecharacteristics, storage characteristics, and battery performances suchas output characteristics, disadvantageously become poor.

A binder distribution coefficient of from 0.5 to 5.0 can be attained byadjusting the conditions for the above-mentioned coating method.Examples of such conditions include selection of a binder, selection ofa solvent for preparing a coating liquid, viscosity of the coatingliquid, solids concentration of the coating liquid, drying method anddrying temperature.

Although the rate of drying is not specifically limited, it is generallypreferred that the rate of drying be low. Further, it is also preferredthat the viscosity and solids concentration of the coating liquid behigh.

The type of metallic current collector is not specifically limited, buta metallic foil is preferably used. In addition, it is preferred thatthe metallic foil have a surface roughness of from 0.1 to 0.9 μm, whichis effective for increasing the adherence between the coatingcomposition and the metallic foil and for improving the high temperaturecharacteristics of the secondary battery of the present invention.

A metallic foil having the above-mentioned surface roughness assumes alusterless appearance. Such a metallic foil can be obtained bysubjecting a metallic foil having a glossy or semiglossy appearance toetching treatment, laser treatment, electroless plating, electrolyticplating, sandblasting or the like to control the surface roughness ofthe metallic foil so that it falls in the range of from 0.1 to 0.9 μm,preferably from 0.2 to 0.8 μm, more preferably from 0.6 to 0.8 μm. Thosecopper foil, nickel foil and the like which are directly obtained byelectrolytic plating and have a surface roughness falling in theabove-mentioned range can also be used as the metallic current collectorin the present invention.

The metallic foil having a surface roughness less than 0.1 μm is notdesirable since an improvement in adherence with the coating compositionis hardly attained. The metallic foil having a surface roughness higherthan 0.9 μm is also not desirable, since such a metallic foil is likelyto suffer a tearing during the coating operation.

With respect to measurement of the surface roughness of a metallic foil,a sample therefor is prepared by cutting a piece of foil (1 cm×1 cm)from the metallic foil. Then, it is placed in a mold, into which anepoxy resin is then poured and hardened. The contents of the mold areallowed to stand at room temperature for one day, and then taken outfrom the mold and cut to expose a cross section of the sample foil. Thesurface of the cross section of the resin including the cross section ofthe metallic foil is polished with a rotating grinder while the crosssection of the metallic foil embedded in the epoxy resin is alsorotated, and then subjected to air blowing. Subsequently, aphotomicrograph of the cross section is taken. The photomicrograph isenlarged and the depths of the concaves on the surface of the metallicfoil are measured, and an average depth obtained therefrom is taken as asurface roughness.

The thickness of the coating composition, comprising a cathode activematerial and a binder, adhering to the metallic foil, is preferably from30 to 300 μm, more preferably from 70 to 130 μm per one side. Examplesof metallic foils to be used for a positive electrode in the presentinvention include an aluminum foil, a nickel foil and a stainless steelfoil, each having a thickness of 5 to 100 μm. Among these foils, analuminum foil of from 8 to 50 μm, preferably from 10 to 30 μm, inthickness is advantageously used.

The thickness of the coating composition, comprising an anode activematerial and a binder, adhering to the metallic foil, is preferably from60 to 750 μm, more preferably from 140 to 400 μm per one side. Examplesof metallic foils to be used for a negative electrode in the presentinvention include a copper foil, a nickel foil and a stainless steelfoil, each having a thickness of from 5 to 100 μm. Among these foils, acopper or stainless steel foil of from 6 to 50 μm, preferably from 8 to25 μm in thickness, is advantageously used.

An adherence test to evaluate the adherence between the active materialparticles and the metallic foil is conducted as follows. Active materialparticles and a binder are applied to a metallic foil, and then driedand compressed to obtain an electrode. The thus obtained electrode issubjected to sizing. A fragment of 2 cm in width and 5 cm in length iscut from the electrode with an NT cutter (manufactured and sold by NTCo., Ltd., Japan) to prepare a test sample.

The active material and binder adhering to the metallic foil arestripped off by 2 cm from the edge of the sample in a longitudinaldirection so that a portion of the surface of the metallic foil isexposed. This portion is attached to a metallic plate by means of astapler to thereby hang the sample.

Next, 80 ml of methanol is placed in a 100 ml glass beaker, which isthen placed in an ultrasonic washer Model: Yamato 2200 manufactured byYamato Co., Ltd., Japan. Tap water is poured into the ultrasonic washerbetween the glass beaker and the inner side wall of the washer to suchan extent that the level of the tap water becomes slightly higher thanthe level of the methanol.

The test sample is placed in the methanol by suspending theabove-mentioned metal plate with a string so that a portion of themetallic foil which has a length of 3 cm and to which the activematerial particles are adhered is completely impregnated with themethanol. The ultrasonic washer is turned on to thereby generateultrasonic waves. Observations of the surface of the coating compositionare continued to see whether a blistering occurs or not with the lapseof time.

With respect to the binder for binding an active material to a currentcollector, there is no particular limitation and, in general, variousorganic polymers can be employed as a binder. Examples of such bindersinclude polyvinyl fluoride, polyvinylidene fluoride, a fluororubber,polyacrylonitrile, polymethacrylonitrile, a nitrile rubber, anethylene-propylene rubber, a styrene-butadiene rubber, polymethylmethacrylate, a polysulfide rubber, cyanoethyl cellulose and methylcellulose.

With respect to the method for using an organic polymer as a binder,various methods can be employed. Examples of methods include a method inwhich the organic polymer is dissolved in a solvent therefor to therebyprepare a binder solution, and an electrode active material is dispersedin the binder solution to thereby prepare a coating liquid, which isused for coating; a method in which an electrode active material isdispersed in an aqueous emulsion of the organic polymer to therebyprepare a coating liquid, which is used for coating; and a method inwhich an electrode active material is preliminarily molded, and asolution of the organic polymer and/or a dispersion of the organicpolymer is applied to the surface of the preliminarily molded material.

Examples of solvents for an organic polymer as a binder includehydrocarbon solvents, such as toluene and hexane; amide solvents, suchas dimethylformamide; ester solvents, such as ethyl acetate; ethersolvents, such as butyl ether; and water. However, the solvent is notlimited to these examples.

The amount of the binder is not particularly limited, but is generallyfrom 0.1 to 20 parts by weight, preferably from 0.5 to 10 parts byweight per 100 parts by weight of the electrode active material.

From the viewpoint of improving high temperature characteristics, it ispreferred that the binder comprise a styrene-butadiene latex having abutadiene content of from 40 to 95% by weight and a gel content of from75 to 100%.

The above-mentioned styrene-butadiene latex can be commercially producedby the conventional techniques of emulsion polymerization. Thestyrene-butadiene latex has a butadiene content of from 40 to 95% byweight, and has a gel content of from 75 to 100%, preferably from 90 to100%, as measured upon drying the styrene-butadiene latex. Theterminology "gel content" used herein means the content of thetoluene-insoluble matter in the polymer.

When the butadiene content of the styrene-butadiene latex is less than40% by weight, the adhesion strength and flexibility of the electrodeare likely to be unsatisfactory. On the other hand, when the butadienecontent is more than 95% by weight, the adhesion strength of theelectrode is likely to be unsatisfactory.

When the gel content of the styrene-butadiene latex is less than 75%,the battery is likely to be unsatisfactory in not only the adhesionstrength of the electrode and the swelling resistance of the electrodeto an electrolytic solution (described later) for use in a non-aqueoustype battery, but also charge retention capability under hightemperature conditions. The exact reason why the gel content of astyrene-butadiene latex polymer affects the charge retention capabilityat high temperatures has not yet been elucidated. However, it ispresumed that the crosslinking degree of the latex polymer, which isrepresented by the gel content thereof, affects the flow characteristicsof the polymer at high temperatures and that the less the flowability ofthe polymer, the smaller the lowering of discharge capacity during ahigh temperature storage.

The styrene-butadiene latex may also contain monomers copolymerizablewith styrene and butadiene. Examples of such copolymerizable monomersinclude ethylenically unsaturated carboxylic acid esters, such as methyl(meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate,(meth)acrylonitrile and hydroxyethyl (meth)acrylate; and ethylenicallyunsaturated carboxylic acids, such as acrylic acid, methacrylic acid,itaconic acid, fumaric acid and maleic acid. With respect to theethylenically unsaturated carboxylic acids, use of a dicarboxylic acid,such as itaconic acid, fumaric acid and maleic acid, is preferred fromthe viewpoint of increasing the adhesive strength of the electrode.Adjustment of the gel content can be performed by conventionaltechniques, such as controlling the polymerization temperature, theamount of an initiator, and the amount of a chain transfer agent.

The particle diameter of the styrene-butadiene latex is not particularlylimited, but is generally from 0.01 to 5 μm, preferably from 0.01 to 0.3μm.

The amount of the styrene-butadiene latex in the coating compositioncontaining an active material is not particularly limited. However, theamount of the latex is generally from 0.1 to 20 parts by weight,preferably from 0.5 to 10 parts by weight per 100 parts by weight of theactive material. When the amount of the latex is less than 0.1 part byweight, a good adhesion strength cannot be obtained. On the other hand,when the amount of the latex is larger than 20 parts by weight, theoverpotential is markedly increased, thereby adversely affecting thecharacteristics of the battery.

The solids concentration of the coating liquid is not particularlylimited, but is generally from 30 to 65% by weight, preferably from 40to 65% by weight.

Further, when a water-soluble polymer, such as a styrene-butadienelatex, is used as a binder, a water-soluble thickener may be added as anadditive thereto in an amount of 2 to 60 parts by weight per 100 partsby weight of the solid value of the styrene-butadiene latex.

As examples of the water-soluble thickner, there can be mentionedcarboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose,ethyl cellulose, polyvinyl alcohol, polyacrylic acid (or polyacrylate),oxidized starch, phosphorylated starch, casein and the like.

In addition, other additives can also be added to the coating liquid.Examples of such other additives include a dispersant, such as sodiumhexametaphosphate, sodium tripolyphosphate, sodium pyrophosphate andsodium polyacrylate, and a nonionic or anionic surfactant as astabilizer for the latex.

When a styrene-butadiene latex is used as a binder for an anode activematerial, the average particle diameter of a carbonaceous material asthe anode active material is preferably in the range of from 0.1 to 50μm, more preferably from 3 to 25 μm, and still more preferably from 5 to15 μm. When the above-mentioned average particle diameter does not fallin the range of from 0.1 to 50 μm, various problems are likely to occur,such as a lowering of the current efficiency, a lowering of thestability of a slurry as the coating liquid comprising the carbonaceousmaterial and the latex, and a rise in the inter-particle resistance inthe coating composition layer of the electrode obtained.

A slurry containing an active material and a latex is applied to asubstrate as a coating liquid, and then dried to form an electrode.Simultaneously with the formation of the electrode, a current collectormay be attached to the electrode. Alternatively, a metallic currentcollector, such as aluminum foil and copper foil, may be used as thesubstrate.

In such coating methods, any of the known coater heads, such as thoseused in the reverse roll method, Comma bar method, gravure method andair knife method, can be used.

The material for the separator is not particularly limited. Examples ofseparator materials include woven fabrics, non-woven fabrics, wovenfabrics of glass fibers, and microporous membranes of synthetic resin.When electrodes comprised of membranes having a large surface area areused, use of such a microporous membrane of synthetic resin,particularly of a polyolefin, as disclosed in, for example, JapanesePatent Application Laid-Open Specification No. 58-59072, is preferredfrom the viewpoint of desired thickness, strength and membraneresistance.

The electrolyte for the organic electrolytic solution is notparticularly limited. Examples of electrolytes include LiClO₄, LiBF₄,LiAsF₆, CF₃ SO₃ Li, (CF₃ SO₂)₂ N•Li, LiPF₆, LiI, LiAlCl₄, NaClO₄, NaBF₄,NaI, (n-Bu)₄ N⁺ ClO₄, (n-Bu)₄ N⁺ BF₄ and KPF₆. The electrolyteconcentration of the organic electrolytic solution is generally fromabout 0.1 to about 2.5M.

Examples of organic solvents usable in the organic electrolytic solutioninclude ethers, ketones, lactones, nitriles, amines, amides, sulfurcompounds, chlorinated hydrocarbons, esters, carbonates, nitrocompounds, phosphoric ester compounds and sulfolane compounds. Of theorganic solvents mentioned above, ethers, ketones, nitriles, chlorinatedhydrocarbons, carbonates and sulfolane compounds are preferred. Morepreferred are cyclic carbonates. Typical examples of cyclic carbonatesinclude tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, anisole,monoglyme, acetonitrile, propionitrile, 4-methyl-2-pentanone,butyronitrile, valeronitrile, benzonitrile, 1,2-dichloroethane,gamma-butyrolactone, dimethoxyethane, methyl formate, propylenecarbonate, ethylene carbonate, vinylene carbonate, dimethylformamide,dimethylsulfoxide, dimethylthioformamide, sulfolane, 3-methylsulfolane,trimethyl phosphate, triethyl phosphate, and mixtures thereof. Organicsolvents are not limited to the above examples.

The structure of the secondary battery is not particularly limited.Examples of battery structures include a paper type battery structurehaving a positive electrode, a separator and a negative electrode in asingle composite layer or a plurality of composite layers, a stack typebattery structure, and a cylindrical battery structure having a positiveelectrode, a separator and a negative electrode wound up in the form ofa roll. It is preferred that the positive and negative electrodes bespirally wound into a structure in which the positive and negativeelectrodes are arranged opposite to each other with a separatortherebetween, and in which each of an innermost wound electrode layerand an outermost wound electrode layer of the spirally wound structurecomprises a layer of the negative electrode. Such a structure ispreferred because deposition of metallic lithium can be markedlysuppressed, thereby effectively preventing a lowering of the batterycapacity which is caused by repeated use, and deterioration of thebattery due to self-discharging and overcharging.

In such a spirally wound structure, it is preferred that a cathodeactive material at each of portions where the winding of the electrodesstarts and terminates, be completely covered by the negative electrodethrough the separator so as not to expose the cathode active material.The negative electrode to be used in this case may be a laminate inwhich two sheets of metallic foils as current collectors, each having ananode active material adhered onto one side thereof, are disposed sothat respective sides of the sheets having no active material thereonare brought into contact with each other. Alternatively, the negativeelectrode may be comprised of a sheet of metallic foil having an anodeactive material adhered onto both sides thereof. The positive electrodeto be used may be of the same structure as that of the negativeelectrode.

When completely covering the cathode active material with the negativeelectrode through the separator, it is preferred that the excessivelength of the negative electrode relative to the length of the positiveelectrode be as short as possible from the viewpoint of decreasing thequantities to be packed. However, when the excessive length of thenegative electrode is designed to be too short, it is likely that aspirally wound structure is produced in which the cathode activematerial is partially exposed. This is due to various factors, forexamples, defects such that the thickness of each electrode is unevenand that a winding machine is unsatisfactory with respect to theprecision in measuring the lengths of the electrodes. Thus, it ispreferred that the positive electrode at each of portions where thewinding starts and terminates, be completely covered by the negativeelectrode through the separator, and that at each of the above-mentionedportions, the distance between the staggered ends of the positive andnegative electrodes which are arranged opposite to each other with theseparator therebetween in the intermediate portion of a spirally woundstructure, be from 1 to 10 mm, more preferably from 2 to 5 mm.

In the secondary battery of the present invention, it is preferred thatthe battery further include as a safety device a PTC (positivetemperature coefficient) element which is operative at a temperature offrom 80° C. to 140° C. and has a sensitive temperature coefficient offrom -10 to -130.

Conventionally, various PTC elements are known. Examples of conventionalPTC elements include BaTiO₃ ceramic type PTC elements. The PTC elementto be used in the present invention is an element which is protectiveagainst overcurrent and overheating and in which an electricallyconductive polymer having PTC characteristics (i.e., characteristicssuch that the electrical resistance is increased with an increase intemperature) is utilized. Examples of PTC elements usable in the presentinvention include various commercially available protective elementswhich are manufactured and sold by K.K. Raychem Japan, Japan under thetrademark of Polyswitch®. The PTC element has a sensitivity to bothtemperature and current and has a function that when either oftemperature and current exceeds a predetermined upper limit, theresistance of the element is automatically raised, thereby shutting offthe current. It is well known in the art that such a PTC element can beincluded in a battery as a safety device. For example, it has alreadybeen a customary practice that a primary lithium battery including sucha PTC element is used, so that when the battery suffers short-circuitingthrough an external circuit, the current is shut off by the action ofthe PTC element, thereby assuring the safety of the battery.

However, Applicants have studied the factors accompanying the occurrenceof overcharging of a secondary battery in detail. As a result, thefollowing facts have been found:

(1) Heat generation is always involved before the overcharged battery isbursted;

(2) Temperature elevation of the battery due to the heat generation isproportionally dependent on the overcharge current; and

(3) The temperature of the battery casing at the time of bursting has acorrelation with the overcharge current, and the larger the overchargecurrent, the lower the temperature of the battery casing as measured atthe time of bursting (it is presumed that a large temperaturedistribution is produced due to a high rate of temperature elevation, sothat a lower temperature value is detected at the casing than thetemperature inside the battery casing).

As apparent from the above findings, safety of the secondary battery ofthe present invention at the time of occurrence of overcharge cannot beeffectively ensured by simply using a fuse which is sensitive only totemperature. On the other hand, when the battery has a fuse which issensitive only to current, current cannot be precisely detected withhigh sensitivity and, therefore, it is impossible to distinguish anormal current from an overcharge current, so that such a fuse also isnot effective to ensure safety of the secondary battery at the time ofoccurrence of overcharge.

As indicated above, the secondary battery of the present invention isgreatly different in behavior at the time of occurrence of overchargefrom conventional batteries. This difference in behavior is ascribed tothe unique combination of active materials for the positive and negativeelectrodes used in the secondary battery of the present invention. Thus,for ensuring the safety of the secondary battery of the presentinvention at the time of occurrence of overcharge, it is preferred thatthe secondary battery be provided with a safety device which issensitive to both temperature and current and has a sensitivetemperature coefficient of a negative value in a specific range. Theterminology "sensitive temperature coefficient" used herein means aparameter which is determined by the method described below and exhibitsa current-dependent sensitive temperature.

Determination of Sensitive Temperature Coefficient

A PTC element is connected to a direct-current power source capable ofconstantly producing a predetermined current and then, heated in an ovento elevate a temperature thereof while applying a predetermined current(A) to the element. When a resistance value of the PTC element becomes1000-fold over the value measured at room temperature, the temperature(°C.) of the PTC element is measured. The above operation is repeatedexcept the current is varied, and the respective temperature of the PTCelement is measured in the same manner as mentioned above, to therebyobtain five temperature values in total. The temperatures (ordinate) areplotted against the currents (abscissa). The sensitive temperaturecoefficient can be obtained as a gradient of the straight line drawn byconnecting the plotted five points.

The temperature at which the PTC element is operative (operativetemperature), means a temperature at which, when a PTC element is heatedwithout flowing current, the resistance value of the PTC element becomes1000-fold over the value measured at room temperature.

The operative temperature of the PTC element to be used in the presentinvention is preferably from 80° to 140° C., more preferably from 85° to140° C. When the operative temperature of the PTC element exceeds 140°C., even if the PTC element operates at that temperature, the secondarybattery continues to produce heat, thereby causing a bursting of thesecondary battery. On the other hand, when the operative temperature ofthe PTC element is less than 80° C., the PTC element is likely towrongly operate at a practically employable temperature.

The sensitive temperature coefficient of the PTC element is preferablyfrom -10 to -130, more preferably from -15 to -100, most preferably from-25 to -80.

When the absolute value of the sensitive temperature coefficient of thePTC element is less than 10, safety at the time of occurrence ofovercharge in the high current range becomes incomplete and, therefore,a bursting of the secondary battery is likely to occur. On the otherhand, when the absolute value of the sensitive temperature coefficientof the PTC element exceeds 130, a practically employable current value,namely, an employable current value at room temperature becomes low and,therefore, the secondary battery having the above-mentioned PTC elementattached thereto cannot be practically used.

In the present invention, the method for attaching the PTC element tothe secondary battery is not particularly limited. For example, the PTCelement can be attached to the secondary battery of the presentinvention within a casing thereof, at a cover for the casing, at a wallof the casing, or the like. As a matter of course, it is desirable thatthe PTC element be at tached to a portion at which the element is moreexactly sensitive to a temperature of the secondary battery. Further, itis noted that when the secondary battery of the present invention isequipped with a PTC element having the above-mentioned specificproperties, safety against overcharging can be advantageously secured inthe entire current range.

In the secondary battery of the present invention, as mentioned above,the water content of the organic electrolytic solution contained in thecasing of the secondary battery is especially important. The terminology"water content" means a water content of the organic electrolyticsolution contained in the casing of the secondary battery which has beenassembled and has not yet been charged. Generally, the organicelectrolytic solution contained in the casing is likely to becontaminated with water from the following origins:

(a) water which has been originally contained in the organicelectrolytic solution;

(b) water which has been originally contained in parts to be assembledinto the secondary battery, such as positive and negative electrodes, aseparator and the like; and

(c) water in air which is likely to be incorporated into the organicelectrolytic solution during the course of assembling the secondarybattery.

With respect to use of the secondary battery of the present invention,there is no particular limitation. The secondary battery of the presentinvention exhibits a high voltage and a high energy density and,therefore, can be advantageously used for, e.g., practicing a method fordriving a portable electronic equipment, which comprises electricallyconnecting the single secondary battery to a portable electronicequipment containing an IC element operative at a voltage of from 2.6 to3.5 V. By the use of the secondary battery of the present invention inthe above-mentioned method, a portable electronic equipment which issmall in size and light in weight, can be provided.

Such portable electronic equipment can be driven at a voltage of from2.5 to 4.2 V, wherein a power consumption is 4 W or less, preferablyfrom 0.5 to 3 W. Examples of such portable electronic equipment includea personal computer which can be driven at a voltage of 3.3 V, anintegrated video camera which can be driven at a voltage of 3.5 V, aportable communication apparatus which can be driven at a voltage of 3.3V.

For use in the above-mentioned electronic equipment, it is preferredthat the capacity of the secondary battery be 400 mAh or more,preferably 700 mAh or more, more preferably from 1500 mAh to 4000 mAh.

When the capacity of the secondary battery is less than 400 mAh, thesecondary battery cannot be continuously used for a long period of time.On the other hand, when the capacity of the secondary battery exceeds4000 mAh, a small and light-in-weight portable electronic equipment canbe hardly realized.

PREFERRED EMBODIMENTS OF THE INVENTION

Hereinbelow, the present invention will be illustrated with reference toExamples, which, however, should not be construed as limiting thepresent invention.

EXAMPLE 1

100 parts by weight of a Li-Co type composite oxide of a composition ofLi₁.03 Co₀.92 Sn₀.02 O₂ were mixed with 2.5 parts by weight of graphiteand 2.5 parts by weight of acetylene black. The resultant mixture wasfurther mixed with a solution which had been prepared by dissolving 2parts by weight of a fluororubber (which is a terpolymer oftetrafluoroethylene, vinylidene fluoride and hexafluoropropylene) in 60parts by weight of a mixed solvent of ethyl acetate and ethyl cellosolvein a weight ratio of 1:1. Thus, a slurry for coating was obtained.

Using a coater equipped with a doctor blade coater head, the obtainedslurry was applied in a thickness of 290 μm to both surfaces of analuminum foil having a width of 600 mm and a thickness of 15 μm.

On the other hand, 100 parts by weight of pulverized needle coke weremixed with a solution which had been prepared by dissolving 5 parts byweight of a fluororubber (which is of the same type as mentioned above)in 90 parts by weight of a mixed solvent of ethyl acetate and ethylcellosolve in a weight ratio of 1:1. Thus, another slurry for coatingwas obtained.

Using a coater equipped with a doctor blade coater head, the obtainedslurry was applied in a thickness of 350 μm to both surfaces of a copperfoil having a width of 600 mm and a thickness of 10 μm.

The above-obtained coated aluminum foil and copper foil wereindividually pressed by means of a calender roll and then, fabricated bymeans of a slitter to form a slit having a width of 41 mm. The aluminumfoil having a coating containing Li₁.03 Co₀.92 Sn₀.02 O₂ (as a cathodeactive material), the copper foil having a coating containing pulverizedneedle coke (as an anode active material) and a microporous polyethylenemembrane (HIPORE 4030U, manufactured and sold by Asahi Kasei KogyoKabushiki Kaisha, Japan) (as a separator) were spirally wound by meansof a winding machine into a spirally wound structure having an outerdiameter of 14.9 mm, wherein the separator is disposed between thealuminum and copper foils. The spirally wound structure was placed in acasing having an outer diameter of 16 mm and then, the spirally woundstructure in the casing was impregnated with an organic electrolyticsolution which had been prepared by dissolving 1M LiBF₄ in a mixedsolvent of propylene carbonate, ethylene carbonate andgamma-butyrolactone in a weight ratio of 1:1:2. The casing was thensealed to thereby obtain a secondary battery having a height of 50 mm,as shown in FIG. 1.

In the accompanying drawing, FIG. 1 is a diagrammatic, verticalcross-sectional view of the secondary battery of the present inventionproduced in Example 1. FIG. 2 is a diagrammatic, cross-sectional viewtaken along line II--II of FIG. 1, showing the spirally wound states ofa positive electrode, a separator and a negative electrode, with casing1 omitted.

In FIG. 1, numeral 1 represents a casing, numeral 2 a sealing cover,numeral 3 a valve hole, numeral 4 a covering plate, numeral 5 a gasvent, numeral 6 a bent brim, numeral 7 a terminal plate, numeral 8 aflexible thin plate, numeral 9 an insulating packing, numeral 10 acutting blade, numeral 11 a positive electrode plate, numeral 12 aseparator, numeral 13 a negative electrode plate, and numeral 14 a pipe.

Referring to FIG. 1, a safety valve device is provided at the topportion of casing 1, namely at sealing cover 2. The safety value deviceis comprised of covering plate 4 bored to have valve hole 3, anddish-shaped terminal plate 7 having gas vent 5 and having its peripheraledge caulking-fitted below bent brim 6 of covering plate 4. The safetyvalue device further comprises a flexible thin plate 8 made of acomposite material of a metallic layer and a synthetic resin layer.Flexible thin plate 8 is securely held at its peripheral edge betweencovering plate 4 and terminal plate 7 and is normally in a state toclose valve hole 3. Cutting blade 10 which is disposed opposite toflexible thin plate 8, is formed by inwardly bending a portion ofterminal plate 7.

With such a structure of the safety valve device, when the pressureinside the battery rises to a predetermined level due, e.g., to anoccurrence of overcharge, the safety valve device is caused to operate,so that flexible thin plate 8 is broken by the action of cutting blade10 and the gas inside the battery is discharged into the air throughvalve hole 3 and then vent 5. Thus, the battery can be prevented frombeing explosively destroyed.

FIG. 2 is a diagrammatic cross-sectional view taken along line II--II ofFIG. 1, showing the spirally wound states of the positive electrode, theseparator and the negative electrode. That is, the positive and negativeelectrodes are spirally wound into a structure such that the positiveand negative electrodes are arranged opposite to each other with theseparator therebetween, wherein each of an innermost wound electrodelayer and an outermost wound electrode layer of the spirally woundstructure comprises a layer of the negative electrode. In such astructure, the surface of the cathode active material of the positiveelectrode is completely covered with the negative electrode through theseparator, thereby preventing the cathode active material from beingexposed. In FIG. 2, for easy understanding of the spirally wound state,the negative electrode is indicated by hatching and the positiveelectrode is indicated by dotting.

The procedure for spirally winding the positive electrode, the separatorand the negative electrode by means of the winding machine is furtherdescribed in detail as follows.

Two reels of separator 12, one reel of positive electrode 11 and onereel of negative electrode 13, namely four reels in total, were set in areel-type automatic winding machine, and winding was conducted under thefollowing winding conditions:

shaft diameter: 4 mmφ,

outer winding length of separator: 66 mm,

second roller length of negative electrode: 370 mm,

second roller length of positive electrode: 340 mm, and

inner winding length of negative electrode: 9 mm.

Conditions, such as an atmosphere for battery assembling and the watercontent of an electrolytic solution, are shown in Table 1.

After assembling, the battery was opened, and the water content of theelectrolytic solution in the battery was measured, and the electrolyticsolution was found to have a water content of 75 ppm. The measurement ofthe water content was conducted using a gas chromatographic apparatus(GC-14A, Shimadzu Corporation, Japan). Porapack Q (1 m×3 φ)(manufactured by Gas Chro Ind., Japan) was employed as a column.

Another secondary battery was sampled from the same lot of products asthat of the above-mentioned battery and charged. As a result, a normalbattery performance was exhibited without occurrence of disadvantageousphenomena, such as expansion of a battery case, as indicated in Table 1.

EXAMPLES 2 TO 6 AND COMPARATIVE EXAMPLES 1 AND 2

Substantially the same procedure as in Example 1 was repeated, exceptthat the operating conditions were varied as indicated in Table 1, tothereby produce secondary batteries of size A.

The thus produced secondary battery in each of the Examples andComparative Examples was opened, and the water content of theelectrolytic solution in the casing was measured. Further, an initialcharge test was conducted with respect to another battery sampled fromthe same lot as that of the above-mentioned battery.

Results are also shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________               Water content  Water content                                                                              Occurrence                             Atmosphere of electrolyt-                                                                        Drying of electrol-                                                                          Current                                                                            of                                     of         ic solution                                                                           step   ytic solution                                                                         efficiency                                                                         expansion                              battery    used for                                                                              before after battery                                                                         at 5th                                                                             of battery                             assembling impregnation                                                                          impregnation                                                                         assembling                                                                            cycle                                                                              casing                                 __________________________________________________________________________    Ex. 1                                                                             Air with                                                                              45 ppm present                                                                               75 ppm 99.6%                                                                              None                                       10% RH                                                                    Ex. 2                                                                             Air with                                                                              15 ppm present                                                                               18 ppm 99.8%                                                                              None                                       50% RH                                                                    Ex. 3                                                                             Air with                                                                              80 ppm present                                                                              135 ppm 99.3%                                                                              None                                       80% RH                                                                    Ex. 4                                                                             Air with                                                                              10 ppm absent 188 ppm 99.5%                                                                              None                                        1% RH                                                                    Ex. 5                                                                             Air with                                                                             250 ppm present                                                                              298 ppm 98.9%                                                                              None                                        1% RH                                                                    Ex. 6                                                                             Air with                                                                             250 ppm present                                                                              320 ppm 98.1%                                                                              None                                       50% RH                                                                    Comp.                                                                             Air with                                                                             250 ppm absent 650 ppm 97.9%                                                                              observed                               Ex. 1                                                                             50% RH                                                                    Comp.                                                                             Air with                                                                              15 ppm absent 480 ppm 98.0%                                                                              observed                               Ex. 2                                                                             50% RH                                                                    __________________________________________________________________________     (Note) RH: Relative humidity                                             

EXAMPLES 7 TO 12

Substantially the same procedure as in Example 1 was repeated, exceptthat when the positive and negative electrode sheets were prepared bycoating, the operating conditions of a coater were varied as indicatedin Table 2, to thereby produce secondary batteries. The binderdispersion coefficients of the above-obtained positive and negativeelectrode sheets are shown in Table 2.

A high-temperature cycle test was conducted at 60° C. with respect tothe thus produced secondary batteries. Results are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                                        Cycle                                                                         test                                                                          at 60° C.                              Drying            Binder distribution                                                                         Capacity                                      conditions        coefficient   at 100th                                      Positive    Negative  Positive Negative                                                                             cycle                                   electrode   electrode electrode                                                                              electrode                                                                            (%)                                     ______________________________________                                        Ex. 7 Hot blast Hot blast 1.88   1.93   92                                          at 120° C.                                                                       at 120° C.                                             Ex. 8 Hot blast Far       0.96   1.18   95                                          at 60° C.                                                                        infrared                                                                      drying                                                        Ex. 9 Far       Hot blast 0.88   1.95   93                                          infrared  at 120° C.                                                   drying                                                                  Ex. 10                                                                              Hot blast Far       6.8    10.5   58                                          at 150° C.                                                                       infrared                                                                      drying                                                        Ex. 11                                                                              Far       Hot blast 0.92   5.1    49                                          infrared  at 150° C.                                                   drying                                                                  Ex. 12                                                                              Air       Air       0.41   0.38   83                                          drying    drying                                                              at 25° C.                                                                        at 25° C.                                              ______________________________________                                    

EXAMPLES 13 TO 16

Secondary batteries were prepared in substantially the same manner as inExample 7, except that copper foils having different surface roughnessesas indicated in Table 3 were individually used as a negative electrodecurrent collector. The binder distribution coefficients of the obtainednegative electrode sheets are shown in Table 3.

Results of an adherence test in which the negative electrode sheets wereimmersed in methanol, and results of a storage test in which theprepared batteries were stored for one month at 60° C. to determine thecapacity retention, are also shown in Table 3.

                  TABLE 3                                                         ______________________________________                                                                           Capacity                                                                      retention                                  Surface      Binder     Adherence  after                                      roughness    distribution                                                                             test by    storage                                    of           coefficient of                                                                           immersing  for one                                    copper       negative   in         month                                      foil         electrode  methanol   at 60° C.                           ______________________________________                                        Example                                                                               0.6μ  1.91       No blister-                                                                            89%                                      13                          ing within                                                                    5 minutes                                         Example                                                                               0.3μ  1.95       No blister-                                                                            87%                                      14                          ing within                                                                    5 minutes                                         Example                                                                              0.01μ  1.90       No blister-                                                                            61%                                      15                          ing within                                                                    1 minute.                                                                     Blistering in                                                                 2 minutes.                                        Example                                                                              0.04μ  1.91       No blister-                                                                            63%                                      16                          ing within                                                                    1 minute.                                                                     Blistering in                                                                 2 minutes                                         ______________________________________                                    

EXAMPLES 17 TO 23

Secondary batteries were prepared in substantially the same manner as inExample 13, except that slurries having compositions described belowwere individually used as a coating slurry for forming a negativeelectrode.

100 Parts by weight of pulverized needle coke were mixed with 10 partsby weight of a styrene-butadiene latex (having a solids content of 50%by weight) prepared according to the respective formulation shown inTable 4, 100 parts by weight of an aqueous solution of carboxymethylcellulose (having a solids content of 1% by weight) as a thickener and 1part by weight of a 1/10N aqueous ammonia, thereby obtaining arespective coating slurry.

Results of an adherence test in which the negative electrode sheets wereimmersed in methanol, and results of a storage test in which theprepared batteries were stored for one month at 60° C. to determine thecapacity retention, are also shown in Table 4.

                                      TABLE 4                                     __________________________________________________________________________    Monomer                   Adherence                                                                            Capacity                                     formulation     Gel Binder                                                                              test by                                                                              retention                                    for latex       content                                                                           distribution                                                                        immersing                                                                            after 1 month                                ST     BD MMA IA                                                                              (%) coefficient                                                                         in methanol                                                                          at 60° C.                             __________________________________________________________________________    Example                                                                            47                                                                              40 10  3 83  1.75  No blister-                                                                          93%                                          17                        ing within                                                                    5 minutes                                           Example                                                                            42                                                                              55 0   3 80  1.81  No blister-                                                                          92%                                          18                        ing within                                                                    5 minutes                                           Example                                                                            33                                                                              60 5   2 98  1.95  No blister-                                                                          95%                                          19                        ing within                                                                    5 minutes                                           Example                                                                            18                                                                              80 0   2 90  1.70  No blister-                                                                          94%                                          20                        ing within                                                                    5 minutes                                           Example                                                                            4 95 0   1 78  1.51  No blister-                                                                          92%                                          21                        ing within                                                                    5 minutes                                           Example                                                                            47                                                                              30 20  3 55  1.78  No blister-                                                                          88%                                          22                        ing within                                                                    5 minutes                                           Example                                                                            0 100                                                                              0   0 80  1.79  Blistering in                                                                        73%                                          23                        2 minutes                                           __________________________________________________________________________     ST: styrene;                                                                  BD: butadiene;                                                                MMA: methyl methacrylate; and                                                 IA: itaconic acid.                                                            Each amount is shown by wt. %.                                           

EXAMPLES 24 TO 29

Secondary batteries were prepared in substantially the same manner as inExample 17, except that a variety of PTC elements shown in Table 5 wereindividually used. The prepared batteries were subjected to overchargetest without voltage limitation. Results obtained are shown in Table 5.

                                      TABLE 5                                     __________________________________________________________________________                     Sensitive                                                                            2A  3A  4A  5A  6A                                              Operative                                                                            temperature                                                                          over-                                                                             over-                                                                             over-                                                                             over-                                                                             over-                                 Element   temperature                                                                          coefficient                                                                          charge                                                                            charge                                                                            charge                                                                            charge                                                                            charge                                __________________________________________________________________________    Example                                                                            PTC  120° C.                                                                       -38.0  ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                         24   element                                                                  Example                                                                            PTC  135° C.                                                                       -51.3  ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                         25   element                                                                  Example                                                                            PTC  118° C.                                                                       -20.1  ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                         26   element                                                                  Example                                                                            PTC  130° C.                                                                       -35.3  ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                         27   element                                                                  Example                                                                            PTC  138° C.                                                                       -75.3  ◯                                                                     ◯                                                                     ◯                                                                     ◯                                                                     ◯                         28   element                                                                  Example                                                                            PTC  130° C.                                                                       -8.0   ◯                                                                     ×                                                                           ×                                                                           ×                                                                           ×                               29   element                                                                  __________________________________________________________________________

EXAMPLE 30

A secondary battery was prepared in substantially the same manner as inExample 1, except that winding conditions were changed as describedbelow. The thus prepared battery and the battery prepared in Example 1were both repeatedly charged and discharged up to 200 cycles, and thenwere allowed to stand for one month at 25° C., permitting the batteriesto self-discharge.

Winding conditions:

Shaft diameter: 4 mm φ

Outer winding length of a separator: 66 mm

Second roller length of a negative electrode: 340 mm

Second roller length of a positive electrode: 370 mm

Inner winding length of a positive electrode: 9 mm (When winding wasconducted under the above conditions, each of the innermost woundelectrode layer and the outermost wound electrode layer of the spirallywound structure became a positive electrode layer.)

Self-discharge ratio:

Example 1: 3%

Example 30: 15%

INDUSTRIAL APPLICABILITY

The secondary battery of the present invention, comprising an organicelectrolytic solution and, disposed therein, a positive electrodecomprised of a lithium-containing composite metal oxide as a cathodeactive material and a negative electrode comprised of a carbonaceousmaterial as an anode active material, wherein the organic electrolyticsolution has a water content of from 5 ppm to 450 ppm, has excellentcurrent efficiency, cycle characteristics, storage characteristics andsafety, and can be advantageously used as a power source for variouselectric devices and electronic devices.

The present invention being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

We claim:
 1. A secondary battery comprising:a casing, an organicelectrolytic solution contained in said casing, said solution having awater content of from 5 to 450 ppm, a positive electrode comprising alithium-containing composite metal oxide as a cathode active material, anegative electrode comprising a carbonaceous material as an anode activematerial, a separator disposed between said positive and negativeelectrodes, and a PTC element which has an operative temperature of from80° C. to 140° C. and has a sensitive temperature coefficient of from-10 to -130, said operative temperature being defined as a temperatureat which a resistance value of the PTC element becomes 1000-fold theresistance value measured at room temperature when the PTC element isheated without flowing current, said sensitive temperature coefficientbeing defined as a gradient of a straight line obtained by plottingtemperatures, at which, when currents of various values are individuallyflowed through the PTC element, resistance values of the PTC elementindividually become 1000-fold the corresponding resistance valuesmeasured at room temperature, against the currents in coordinates ofcurrent (abscissa) and temperature (ordinate), said positive andnegative electrodes and separator being disposed in said organicelectrolytic solution, wherein at least one of said positive andnegative electrodes is in the form of a coating composition formed on ametallic current collector, said coating composition comprising anactive material corresponding to said respective electrode and a binder,said metallic current collector being formed of a metallic foil having asurface roughness of from 0.1 to 0.9 μm.
 2. The secondary batteryaccording to claim 1, wherein binder is distributed in said coatingcomposition at a binder distribution coefficient of from 0.5 to 5.0. 3.The secondary battery according to claim 1 or 2, wherein said bindercomprises a styrene-butadiene latex having a butadiene content of from40 to 95% by weight and a gel content of from 75 to 100%.
 4. Thesecondary battery according to claim 1, wherein said organicelectrolytic solution comprises at least one organic solvent and anelectrolyte dissolved therein, said at least one organic solvent beingselected from the group consisting of an ether, a ketone, a lactone, anitrile, an amine, an amide, a sulfur compound, a chlorinatedhydrocarbon, an ester, a carbonate, a nitro compound, a phosphoric acidester and a sulfolane compound.
 5. The secondary battery according toclaim 1, wherein said positive and negative electrodes are spirallywound into a structure such that said positive and negative electrodesare arranged opposite to each other with said separator therebetween,wherein each of an innermost wound electrode layer and an outermostwound electrode layer of said spirally wound structure comprises a layerof said negative electrode.
 6. A method for preventing gas generationand rapid temperature elevation at overcharge of a secondary battery,thereby ensuring safety of the secondary battery, whichcomprises:providing a PTC element such that said PTC element has anoperative temperature of from 80° C. to 140° C. and a sensitivetemperature coefficient of from -10 to -130, said operative temperaturebeing defined as a temperature at which a resistance value of the PTCelement becomes 1000-fold the resistance value measured at roomtemperature when the PTC element is heated without flowing current, saidsensitive temperature coefficient being defined as a gradient of astraight line obtained by plotting temperatures, at which, when currentsof various values are individually flowed through the PTC element,resistance values of the PTC element individually become 1000-fold thecorresponding resistance values measured at room temperature, againstthe currents in coordinates of current (abscissa) and temperature(ordinate), and operably connecting said PTC element to a secondarybattery, said secondary battery comprising: a casing, an organicelectrolytic solution contained in said casing, said solution having awater content of from 5 to 450 ppm, a positive electrode comprising alithium-containing composite metal oxide as a cathode active material, anegative electrode comprising a carbonaceous material as an anode activematerial, and a separator disposed between said positive and negativeelectrodes, said positive and negative electrodes and separator beingdisposed in said organic electrolytic solution, wherein at least one ofsaid positive and negative electrodes is in the form of a coatingcomposition formed on a metallic current collector, said coatingcomposition comprising an active material corresponding to saidrespective electrode and a binder, said metallic current collector beingformed of a metallic foil having a surface roughness of from 0.1 to 0.9μm.
 7. The secondary battery according to claim 1, wherein said PTCelement has a sensitive temperature coefficient of from -15 to -100. 8.The secondary battery according to claim 7, wherein said PTC element hasa sensitive temperature coefficient of from -25 to -80.
 9. The secondarybattery according to claim 7, wherein said PTC element has an operativetemperature of from 85° to 140° C.
 10. The secondary battery accordingto claim 1, wherein said lithium-containing composite metal oxide is acompound represented by the formula Li_(x) Co_(y) N_(z) O₂ wherein N isat least one member selected from the group consisting of Al, In and Sn,x is a number from 0.05 to 1.10, y is a number from 0.85 to 1.00, and zis a number from 0.001 to 0.10.
 11. The secondary battery according toclaim 1, wherein said carbonaceous material has a BET specific surfacearea A (m² /g) in the range of 0.1<A<100 and has a crystal thickness Lc(Å) and a true density ρ (g/cm³) that satisfy the relationships:

    10<Lc<120ρ-189

and

    1.70<ρ<2.18.


12. The secondary battery according to claim 10, wherein saidcarbonaceous material has a BET specific surface area A (m² /g) in therange of 0.1<A<100 and has a crystal thickness Lc (Å) and a true densityρ (g/cm³) that satisfy the relationships:

    10<Lc<120ρ-189

and

    1.70<ρ<2.18.


13. The secondary battery according to claim 1, wherein said metallicfoil has a surface roughness of from 0.6 to 0.8 μm.
 14. The secondarybattery according to claim 1, wherein said metallic foil is contained insaid positive electrode and is made of aluminum, nickel or stainlesssteel.
 15. The secondary battery according to claim 1, wherein saidmetallic foil is contained in said negative electrode and is made ofcopper, nickel or stainless steel.
 16. The secondary battery accordingto claim 2, wherein said binder is distributed in said coatingcomposition at a binder distribution coefficient of from 0.75 to 2.0.17. The secondary battery according to claim 1, wherein said organicelectrolytic solution contains an electrolyte selected from the groupconsisting of LiClO₄, LiBF₄, LiAsF₆, CF₃ SO₃ Li, (CF₃ SO₂)₂ N·Li, LiPF₆,LiI and LiAlCl₄.